Have plugin parameter for external links

[thomasgriffin]

As suggested by Travis Smith (wp_smith), it would be good to have an optional parameter for each plugin, 'external_url' which would override the default plugins API/no link setup. They could link the plugin to an external resource where the plugin could be purchased (say for privately purchased plugins like Gravity Forms). The trick would be to make sure that the developer has the correct slug entered. That can't be monitored on our part, but we can make sure they understand it.